The present invention relates generally to abrasive or superabrasive tools. In particular, the present invention relates to a rotatable grinding wheel having an abrasive or superabrasive surface.
Certain types of workpieces (plastic and glass lenses, stone, concrete, and ceramic, for example) can be advantageously shaped using grinding tools, such as a wheel or disc, which have an abrasive work surface, particularly a superabrasive work surface, a superabrasive surface also being an abrasive surface but having a higher abrasivity. The work surface of the grinding tool can be made up of an abrasive band around the outer circumference of the wheel or disk. The work surface usually includes particles of super hard or abrasive material, such as diamond, cubic boron nitride, or boron suboxide surrounded by a bond material and/or embedded in a metal matrix. It is these abrasive particles that primarily act to cut or grind a workpiece as it is brought into contact with a rotating work surface of the grinding tool.
It is known to form cutting or grinding wheels comprising segments of abrasive material. The abrasive segments can be formed by mixing abrasive particles such as diamonds and metallic powder and/or other filler or bond material in a mold and pressure molding the mixture at an elevated temperature. Forming abrasive segments in this way, however, can create areas having high concentrations of hard or abrasive particles and areas having low concentrations of abrasive particles in the segment. Further, the concentration of abrasive particles at an abrasive surface affects grinding characteristics of the wheel such as wheel wear rate and grinding rate. As such, non-uniform or randomly varying concentrations of abrasive particles can cause unstable cutting or grinding performance. Also, forming abrasive segments in this way can be relatively expensive because a relatively high number of abrasive particles are used.
To reduce problems associated with non-uniform or randomly varying concentrations of abrasive particles in abrasive surfaces, it is known to form abrasive segments in which concentrations of abrasive particles vary in an orderly manner. For example, abrasive segments can be formed having substantially parallel, planar layers of abrasive particles separated by regions of bond material. Abrasive material having such layers of abrasive particles are disclosed in, for example, U.S. Pat. No. 5,620,489, issued on Apr. 15, 1997 to Tselesin, entitled Method for Making Powder Preform and Abrasive Articles Made Therefrom; U.S. Pat. No. 5,049,165, issued Sep. 17, 1991 to Tselesin entitled Composite Material; and Japanese Laid Open Patent Publication J.P. Hei. 3-161278 by Tanno Yoshiyuki, published Jul. 11, 1991 for Diamond Saw Blade (xe2x80x9cYoshiyukixe2x80x9d).
Yoshiyuki discloses a saw blade for cutting stone, concrete, and/or fire resistant material. The saw blade is formed from abrasive segments having planar layers of abrasive particles. The layers of abrasive particles are aligned with a direction of rotation of the saw blade such that the cut in a workpiece forms grooves, as can be seen in FIG. 3 of Yoshiyuki. Such grooves are formed because the areas of bond material between planes of abrasive particles wear faster than the areas of the planes of abrasive particles.
However, for some applications of a grinding tool, wear grooves are undesirable or unacceptable. In some cases, it is specifically desirable to be able to produce a smooth, rounded edge on a workpiece. For example, a type of grinding wheel, known as a pencil wheel, is generally used to grind the edges of panes of glass to remove sharp edges of the glass and leave rounded edges free of cracks that could cause the glass to break. The production of grooves in the rounded edge would be undesirable.
In addition to the foregoing, an improvement over the generally practiced methods of assembling grinding wheels is desired. Typically, assembly of a grinding wheel includes either a brazing or a sintering process in order to bond the abrasive material to the support plate(s). These processes may be disfavored for a number of reasons. For example, brazing an abrasive layer to an aluminum support plate (a preferred material due to its light weight) may be difficult to accomplish due to the presence of aluminum oxide on the surface of the support plate which inhibits wetting-out of the braze material. Sintering is generally disfavored due to the long time period and high temperature required. Furthermore, both sintering and brazing are incompatible with non-metallic (e.g., polymeric) support plates. In view of these disadvantages, an improved method of bonding the abrasive layer to the support plate(s) in a grinding wheel is desired.
In accordance with the present invention, a grinding wheel exhibits an abrasive surface having an ordered concentration of abrasive particles to advantageously produce stable grinding results. But also, the abrasive surface of the wheel is able to produce a smooth edge on a workpiece. In some instances, the edge produced on a workpiece may also be rounded.
The present invention includes a generally cylindrical abrasive grinding wheel which is rotatable about an axis of rotation. A substantially cylindrical region of abrasive material having an abrasive surface on an outer peripheral surface thereof is formed from a plurality of layers of abrasive particles. Each layer of abrasive particles extends in at least a circumferential direction and a radial direction of the cylindrical region of abrasive material. By extending the layers in a radial direction, as an edge of an abrasive particle layer is worn away by use of the wheel, a fresh edge will advantageously be exposed. When a wheel having a shaped or profiled edge is used, however, the edge may have to be re-profiled as it is worn down.
One aspect of the invention is characterized by the layers of abrasive particles being arranged on the abrasive surface such that any circular path defined by an intersection of a plane perpendicular to the axis of rotation of the grinding wheel and a complete circumference of the abrasive surface will intersect at least one of the plurality of layers of abrasive particles.
Another aspect of the invention can be characterized by the layers of abrasive particles being tilted with respect to the axis of rotation of the grinding wheel to form an angle of between 0 degrees and 180 degrees, exclusive, therewith. In this way, as the grinding wheel is rotated through a 360 degree rotation, an exposed edge of a single abrasive particle layer will sweep over an axial distance wider than the width of the exposed edge of the abrasive particle layer. If the layers of abrasive particles are tilted with respect to the axis of rotation such that the width of the axial distances over which each abrasive particle layer sweeps meet or overlap, then grooving on the surface of a workpiece can be reduced and preferably eliminated.
Yet another aspect of the invention can be characterized by the grinding wheel being formed from a plurality of abrasive segments each including layers of abrasive particles. The layers of abrasive particles are staggered in an axial direction from one segment to another. In this way, the exposed edges of the abrasive particle layers will sweep across a greater portion of an axial thickness of the abrasive surface. This can also reduce grooving on a workpiece. In some embodiments, it may be feasible to reduce grooving with segments whose abrasive particles are not in layers but are randomly spaced.
Yet another aspect of the invention can be characterized by the grinding wheel including a layer of metal bond abrasive which is adhesively bonded to at least one support plate. As used herein the term xe2x80x9cadhesivexe2x80x9d refers to a polymeric organic material capable of holding solid materials together by means of surface attachment. As used herein the term xe2x80x9cmetal bond abrasivexe2x80x9d refers to an abrasive material comprising a plurality of abrasive particles distributed throughout a metal bond material. The abrasive particles may be randomly distributed (i.e., non-uniform or randomly varying concentrations) throughout the metal bond material or the concentration of abrasive particles may vary in an orderly manner (e.g., substantially parallel, planar layers of abrasive particles separated by regions of metal bond material). The layer of metal bond abrasive may comprise a single mass or more than one mass. In a preferred embodiment, a plurality of discrete metal bond abrasive segments are circumferrentially spaced between two support plates and are adhesively bonded to the support plates by a structural adhesive which is interposed between the abrasive segments and the support plates.